Background: Cardiovascular side effects of
clozapine are not uncommon, but few systematic studies of these
effects have been performed. In this study, we reviewed data on
the electrocardiographic (ECG) abnormalities in patients treated
with clozapine.

Method: Sixty-one patients treated with
clozapine were selected from the Seoul National University
Hospital Treatment-Resistant Schizophrenia Clinic. A retrospective chart review was conducted to identify ECG
abnormalities and cardiovascular side effects.

Results: The prevalence of ECG abnormalities in
patients who had been using antipsychotics other than clozapine
was 13.6% at baseline, which increased significantly to 31.1%
after commencement of clozapine treatment. Among the 53 patients
without baseline ECG abnormalities, 13 showed new-onset ECG
abnormalities after using clozapine. Normal ECG under previous
antipsychotic medication reduced the risk of new-onset ECG
abnormalities, whereas increased age was found to increase the
risk. The occurrence of orthostatic hypotension or tachycardia
was not related to the development of ECG abnormalities. Most of
the newly developed abnormalities had little clinical
significance, and they tended to occur during the initial phase
of treatment. In 10 patients, ECGs normalized despite the
continued use of clozapine. Clozapine increased corrected QT
interval (QTc) in a dose-dependent fashion; however, the clinical
significance of this observation is uncertain. Pathologic
prolongation of QTc was found to be rare.

Conclusion: Although a substantial portion of
patients treated with clozapine developed ECG abnormalities, most
of the abnormalities were benign and did not hinder further
treatment.